Electronic devices for ultra-high frequency microwave signals (&gt;10 GHz) are difficult to design because interconnections have unintentional capacitance and inductances, causing undesirable side effects. Dissimilar families of microwave electronic devices, desirable approaches in themselves, become an extremely difficult problem to put together without causing parasitic distortions of the signal.
At microwave frequencies there are no simple interconnects to be used in integrated circuits. Simple low frequency interconnects show dispersion, attenuation, and phase shift at microwave frequencies and therefore have to be designed and treated as transmission lines. There are a number of popular transmission line geometries available for microwave circuits. The simplest and most widely used structure is shown in FIG. 2. This structure is known as a microstrip. (See T. C. Edwards, Foundations for Microstrip Circuit Design, John Wiley and Sons, 1981.) A microstrip consists of a metal strip of controlled width on the surface of the semiconductor or ceramic substrate. The other side of the substrate is completely metalized and forms the microstrip ground plane. Another transmission medium used in microwave circuits is known as coplanar waveguide (CPW) which is shown in FIG. 1. The difference between CPW and microstrip is that CPW has all the conductors including the ground plane on the same side of the substrate adding the advantage of easier access to ground.
Microstrip and CPW are generally not combined on the same monolithic circuit. But it is desirable to be able to connect CPW circuits to microstrip circuits in order to form larger subsystems.